Cathode-ray oscilloscope, electronic-display device containing a cathode-ray tube (CRT) that generates an electron beam that is used to produce visible patterns, or graphs, on a phosphorescent screen. The graphs plot the relationships between two or more variables, with the horizontal axis normally being a function of time and the vertical axis usually a function of the voltage generated by the input signal to the oscilloscope. Because almost any physical phenomenon can be converted into a corresponding electric voltage through the use of a transducer, the oscilloscope is a versatile tool in all forms of physical investigation. The German physicist Ferdinand Braun developed the first cathode-ray oscilloscope in 1897.

A cathode-ray oscilloscope.

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Speed of response is the cathode-ray oscilloscope’s chief advantage over other plotting devices. General-purpose oscilloscopes have plotting frequencies of up to 100 megahertz (MHz), or 100 million cycles per second. Response times as rapid as 2,000 MHz are achievable with special-purpose high-speed oscilloscopes.

The central component in this device, the cathode-ray tube, consists of an evacuated glass container with a phosphorescent coating at one end (similar to that of a television screen) and an electron gun and a system for focusing and deflecting the beam of electrons at the other. The electron beam emerging from the electron gun passes between pairs of metal plates mounted in such a way that they deflect the beam horizontally and vertically to control the production of a luminous pattern on the screen. The screen image is a visual representation of the voltages applied to the deflection plates. Alternatively, the beam may be deflected magnetically by varying the currents through externally mounted deflection coils. Thus, almost any graph can be plotted on the screen by generating horizontal and vertical deflection voltages or currents proportional to the lengths, velocities, or other quantities being observed.

It is sometimes necessary or desirable to plot more than one waveform at the same time on the screen of an oscilloscope. With the use of a variety of techniques, four or more plots can be simultaneously shown. With a dual-trace amplifier and a single electron gun, two signals may be shown at what appears to be the same time. Actually, the amplifier electronically switches rapidly between the two observed signals. In a split-beam CRT the electron beam from a single gun is split, with the two parts receiving different vertical deflections. A dual-gun CRT uses two separate electron guns, each having its own focus and brightness controls. By combining two dual-trace amplifiers with a dual-gun CRT, four individual plots can be obtained.

The cathode-ray oscilloscope is one of the most widely used test instruments; its commercial, engineering, and scientific applications include acoustic research, television-production engineering, and electronics design.